The primary aim of this protocol is to: 1) develop a registry for herediatary hemorrhagic telangiectasia (HHT) to characterize systematically the clinical presentation, the prevalance of particular manifestations and the natural history of this disorder, linking these phenotypic subtypes to particular genetic defects that have been and are in the process of being identified and; 2) to determine the prevalance of pulmonary arteriovenous malformation (pAVM) in asymptomatic patients with HHT, to study if certain patient subgroups or families are at increased risk of pAVM and to document the sensitivity and specificity of commonly used screening tests for the detection of pAVM.